Exploring Local Dark Matter with the Space Interferometry Mission (SIM PlanetQuest)From Quantum to Cosmos: Fundamental Physics in Space for the Next Decade

Figure courtesy of B. Gibson (Central Lancashire)Steven Majewski (Univ.Virginia)

Authors of Recent SIM Local Dark Matter White PaperSteven Majewski (Univ. Virginia) James Bullock (UC-Irvine) Andreas Burkert (Univ.-Sternwarte Munich) Brad Gibson (Univ. Central Lancashire) Oleg Gnedin (Univ. Mich) Eva Grebel (Astron. Rechens-Institut, Univ. Heidelberg) Puragra Guhathakurta (UC-Santa Cruz) Amina Helmi (Kapteyn Astron. Institute, Groningen) Kathryn Johnston (Columbia Univ.) Pavel Kroupa (Argelander Inst. for Astronomy, Univ. Bonn) Manuel Metz (Argelander Inst. for Astronomy, Univ. Bonn) Ben Moore (Inst. For Theoretical Physics, Univ. Zurich) Richard Patterson (Univ. Virginia) Ed Shaya (Univ. Maryland) Louis Strigari (UC-Irvine) Roeland van der Marel (STScI)

Growth of Structure in a Cold Dark Matter UniverseAnimation by Ben MooreUniversity of Zurich

Numerical simulations make rich variety of predictions about structure and dynamics on galactic to largest scales. Great success in matching observations on largest scales. But numerous problems matching data on galaxy scales, e.g.: missing satellites problem/mass spectrum of subhalos central cusps problem angular momentum problemsAbadi et al. (2003): Current cosmological simulations have difficulties making anything that looks like a real galaxy.

Thus a current focus for advancing DM theory is attempting to resolve problems on small (galaxy) scales. So understanding/explaining dynamics of Local Group, Milky Way and satellite system are central to progress in DM theory, hierarchical formation, galaxy evolution.

Microscopic nature of DM affects the way it clusters around galaxies, thus can be probed by exploration of LG & MW. Deriving a globally self-consistent MW DM halo model will provide information on the mass range and dissipational properties of the dark matter particle.

Microscopic nature of DM affects the way it clusters around galaxies, thus can be probed by exploration of LG & MW. Deriving a globally self-consistent MW DM halo model will provide information on the mass range and dissipational properties of the dark matter particle. useful information for experiments that aim to detect DM particle directly on (inside) Earth.CDMSII (Berkeley) Soudan Mine, MN

LUX (Brown Univ.) -- Homestake Mine, SDXENON (Columbia Univ.) Gran Sasso Massif, Italy

1) Measure shape/orientation/density law/lumpiness of MW potential w/tidal streams (SIM far, Gaia close)2) Measure shape/orientation of galaxy potentials withhypervelocity stars (SIM) 3) Mapping late infall via orbits of satellites (SIM) 4) Measure ang. momentum distn/anisotropy/orbits of MW stars, clusters (SIM far, Gaia close)5) Measure DM temperature by mapping DM phasespace density (i.e. cusp vs. core) in dSph (SIM) 6) Local Group dynamics (Shaya talk) (SIM) Astrometric experiments to measure galactic dynamics, structure, local dark matter in SIM/Gaia era

Model expectations: more oblate (q ~ 0.6) more typicalHalo Shape CDM predicts DM halos to be trixial, but rounder at larger radii.

Dwarf Galaxy vs. Milky Way-like SystemAnimation by Kathryn JohnstonColumbia UniversityTidal Tails Are Very Sensitive Galactic Mass Probes

NGC 5907: Modeling the Tidal Disruption Extragalactic systems: With no RVs and only on-sky projection, left with degeneracies of orbital precession, ellipticity, halo shape, etc but should not be problem inside of Milky Way Martinez-Delgado et al. (2008)Tidal Tails Are Very Sensitive Galactic Mass Probes

Early work on 2-D data (i.e. great circle of presumed Sgr carbon stars & 2MASS M giants) suggested Galactic DM halo ~ spherical (Ibata et al. 2001, 2003, Majewski et al. 2003).Application in the Milky WayMajewski et al. 2003, Law et al. 2005

Helmi (2004) with 3 phase space coordinates (spatial positions + RVs) finds need for prolate halo. Application in the Milky Way

~50 kpcJohnston, Law & Majewski (2005) - 4 coords (3 space + RV): Gives only slightly oblate halo to ~50 kpc (q ~ 0.92 +/- 0.2).

But some problems with matching leading arm velocities unresolved.

Strongly rules out prolate (5s): Precesses Sgr backwards.

Tidal Tails Are Very Sensitive Galactic Mass Probes Experiment requires 6-D phase space information for stream stars. Requires SIM-accurate proper motions for faint, distant stars (e.g., < ~10 as/yr at V ~18 for ~100 kpc giant stars). Gaia useful for nearby (~10 kpc) streams.Animations by C. Moskowitz & K. Johnston (Wesleyan University)correct potentialincorrect potential

Sloan Digital Sky SurveyFrom Carl Grillmair, in Unwin et al. (2007)Now finding many lower surface brightness streams in the Milky Way halo with starcounts and radial velocity surveys.

Needed: Proper Motions at mas/yr LevelSIM PlanetQuest: - 4 as/year for V~ 15-20 (giant) stars - For 100s of pre-selected tidal stream targets expect 1% accuracy on halo flattening and QLSR - Milky Way mass profile from multiple streams.MILKY WAY MASS PROFILE FROM MULTIPLE STREAMS. Gaia could do only for nearby (few 10 kpc) streams.

Hypervelocity Stars Brown et al. (2005, 2006): Half-dozen stars w/Galactocentric velocity = 550-720 km/sHills (1998), Yu & Tremaine (2003): Only known mechanism: ejection from deep potential of SMBH

Gnedin et al. (2006): Modeling HVS SDSS J090745.0+024507Milky Way: q1/q3 = 0.9, q2/q3 = 0.7 (triaxial, prolate) Most of halo shape sensitivity in transverse velocity at large r.Deviation in transverse velocity by non-spherical potential

True distance of HVS cleanly determined from sm = 100 mas yr-1. Constraints on orientation of triaxial halo with sm = 20 mas yr-1. Constraints on axial ratios with sm = 10 mas yr-1. Known HVSs have V = 16-20 (SIM territory)XGC major axisIf MS70 kpcIf BHB40 kpcYGCmajor axisZGC major axisZGC major axisYGCmajor axisXGC major axisHVS SDSS J090745.0+024507

Can the method be generalized? Existence of an HVS from LMC recently reported Gualandris et al. (2007), Bonanos (2008) Deriving full 3-D trajectory would pin down the location of massive black hole in LMC. Numerous M31 HVSs expected, including 1000s within virialized halo of MW (Sherwin et al. 2008). Tell us about M31 halo? Mass distribution of Local Group? Must have as astrometry at faint mags -- SIM only

The Milky Way Then and NowCDM models suggest that Milky Way of today: Is very lumpy - should have numerous subhalos/satellites.0.4 billion years old13.4 billion years oldCourtesy Ben MooreUniversity of Zurich

Mass spectrum of subhalos is a function of DM physics Mass spectrum ~ M-1 (Dieman et al. 2008), but cut-off mass function of particle nature of DM. If DM = cold (e.g., WIMPS), minimum mass = earth mass, number of subhalos ~ 1013. If DM = warm (e.g., sterile ), minimum mass = 108 Msun, number of subhalos ~ < 100.

(Stadel et al., in prep.)

In either case, where are the missing satellites? Possibly mainly DARK. Only most massive dozen or so lumps form stars (red lumps above)? Visible satellites represent only tips of the dark matter icebergs?

Moore et al. (1999), Kaufmann et al. (1993), Klypin et al. (1999)massNumber of subhalos

Measuring Halo (Dark) Lumpinesse.g., Johnston, Spergel & Haydn (2002)perturbation of circular orbits in halo with 256 lumpsAfter 1.3 GyrAfter 2.6 GyrAfter 4 Gyrangulardeviationsvelocitydeviationsangulardeviationsvelocitydeviationsangulardeviationsvelocitydeviations

Sensitivity of test increases with long cold streamsGrillmair (2006) and 6-D data (SIM): For example, perturbation points in streams should be identifiable with trace back of stream star orbits. Rockosi et al. (2002), Odenkirchen et al. (2001,2003)

Testing Hierarchical Formation and Late Infall Infall of DM onto MW leaves fingerprint in the orbits of satellite galaxies, any accreted globular clusters and halo stars. Models point to infall along filaments.(Moore et al. 2001)

z = 10z = 0(Moore et al. 2006)Evolution of luminous subhalos in a MW galaxy: Surviving galaxy satellites of today (boxes) were most distant subhalos at z = 10, last to fall into MW. Earlier infall came from closer matter, and luminous parts now spread out among the debris (stars and star clusters) of halo. In either case, orbital shapes/correlations tell us how infall proceeded at corresponding infall epoch. Kinematics of late and early infall expected to differ.

Current Milky Way satellites show strong spatial anisotropy and hint at evidence for correlated orbits: Orbital poles for MW satellites (Palma, Majewski & Johnston 2003) Infall in a few groups of DM subhalos? Break-up of formerly larger satellites? Formed as tidal dwarfs?

To derive transverse velocities good to 10 km/s requires: ~ 10 as for satellites at ~250 kpc (Leo I, II, CanVen) for V ~ 19.5 giant stars (SIM only) ~ 20 as for satellites at ~100 kpc (UMi, Dra, Car, ) for V ~ 17.5 giant stars (SIM or Gaia many star average) Gaia cannot play this game for many of the newfound ultra-low luminosity dSphs (even close ones) because there are few/no member stars bright enough:(Belokurov et al. 2007)

z = 10z = 0(Moore et al. 2006)CDM predicts halo anisotropy gradient (more radial at larger r)To test, need in situ measures of halo star orbital anisotropy:Similar proper motion requirements as for dSphs, but single stars.Gaia relegated only to inner halo here.Few 100 stars to 5 km/s (compared to RV dispersion ~100 km/s)

Determining the Nature of Dark Matter with SIM CDM: potentially ruinous difficulties on small scales: Missing satellites problem Angular momentum/too small disks problem Cusps predicted, but rotation curves prefer cored profiles, and luminous matter profiles are cored.CDM:WDM:High primordial phase space densityLow primordial phase space densityCuspy NFW profilesCored density profilesWIMPS: e.g., axions, neutralinos, e.g., gravitinos, light sterile s RegionProbedbydSphstars New Test: Stellar ms in M.W. dSphs.

Determining the Nature of Dark Matter with SIM Log-slope of dark matter density profile Velocity Anisotropy of Stars Future: 200 proper motions at ~5 km/s with SIM will break this degeneracy. Measure log-slope of DM density profile at stellar radius to 0.2. Discriminate between viable WDM and CDM at the ~3 sigma level.

Strigari et al. (2007, 2008) MW dSphs ideal for testing nature of Dark Matter. But currently: Radial velocity studies have strong degeneracy between DM density slope and stellar velocity anisotropy. Even with 1000s of RVs, cant distinguish cored from cusp halos (WDM vs CDM).With SIMWithout SIMLeo I

Determining the Nature of Dark Matter with SIMStrigari, Bullock, Kaplinghat, Kazantzidis, Majewski & Munoz 2008 ~100 days of SIM time (~key project) will provide approximately 200 stars in Draco dSph to V = 19 with 5 km/s transverse velocities (sufficient).Error in measured slopeAssuming < 7 km/s errors =< 20 mas at 80 kpc (Draco)luminosity function3 km/s5 km/s7 km/s10 km/s

Local Group Dynamics with SIM (Ed Shaya Talk) Local Group: ms of ~30 galaxies in the Local Group. Constrain LG matter distribution Proper motions key to constraining mass on ~ 5 Mpc scale. Positions/orbits of galaxies back in time, masses of individual galaxies. Test cosmological expectationsShaya et al.

Growth of Structure in a Cold Dark Matter UniverseCDM Galaxy Merger Tree (Wechsler et al. 2002) Since Searle & Zinn (1978) notion of accretion, including late infall, a central question of Milky Way (MW) formation studies. Merging also a key element of galaxy formation models with CDM.time

Gnedin et al. (2006): Modeling HVS SDSS J090745.0+024507Milky Way: q1/q3 = 0.9, q2/q3 = 0.7 (triaxial, prolate) Most of halo shape sensitivity in transverse velocity at large r.

BootesBelokurov et al. 2006Munoz et al. 2006Can Ven IZucker et al. 2006Ursa MajorWillman et al. 2005 or has the problem just been one of accounting??About a dozen or more recent discoveries:

Where are the missing satellites? Are there enough discoveries to fill the gap? Doesnt fix shortfall at all massesMoore et al. (1999), Kaufmann et al. (1993), Klypin et al. (1999)massnumber

Sagittarius Debris Stream Dynamically Cold For ~2 Gyr If sv all from scattering, Sgr tail hotter than expected for smooth halo however, consistent with influence of just one LMC-like lump. Cannot yet rule out some lucky lumpier halos. But note, some dispersion is intrinsic to Sgr. Longer Sgr,, initially colder streams, and/or 6-D data will yield more definitive results.

Trailing arm data from Ibata et al. (1997), Majewski et al. (2004, 2007)rlimL, orbital longitude (deg)

Animation by James Bullock & Kathryn Johnston (2005)Streams shown to = 38 mag/arcsec2. Today ~1 stream with < 30 mag/arcsec2 should be visible per MW-like galaxy. (Johnston et al., in prep.)HierarchicalMerging Seen on Galactic ScalesBullock & Johnston 2005

Known Milky Way StreamsBullock & Johnston ModelGrowing Convergence of Stream Data and Models

okInternational team, theorists and observers, established but also young contributorsThis growth is demonstrated by this CDM simulation by Ben Moore Can see how DM lumps assemble larger lumps as they fall along filamentary structuresWhen such smaller systems encounter the stronger potential of a galaxy like the MW, tidal forces are expected to act upon them to strip Out stars, which then become part of the larger system. Each orbit around the parent galaxy contributes to the formation of extended tails of debris, shown here by different colors corresponding to successive orbital stripping

Unfortunately, this is an area of SOME CONTROVERSYBut more recently, Amina Helmi has put forward the SURPRISING HYPOTHESIS that our latest Sgr velocity data require a PROLATE dark halo!In our own analysis of the orbital plane precession using the largest sample of Sgr data we findIn such a relatively non-lumpy potential, the streams become extremely nice probes of the overall mass distribution of the Galaxy.To do the experiment right requires full 6-D phase space data, which means having accurate proper motions for rather faint and distant stars.However, the needed proper motions are in the unique reach of SIM and we have a Key Project experiment scheduled for SIM meant to address this very problem. However, some MORE BAISC knowledge about the Galactic potential can be deduced from the current 4-D data. These are issues of great interest because the CDM models I showed earlier make SPECIFIC PREDICTIONS about what we should expect to see.. Hundreds or thousands of subhalos, both dark and luminous.First let me address the lumpiness issue, where some interesting results are found. Halo lumpiness is nicely probed by tidal streams as shown by these simulations by Johnston et al. of dynamically cold streams of stars placed in a halo with a large numberof lumps. As may be seen, encounters with lumps will, with time, act to perturb the dynamically cold stream giving rise to both positional and velocity deviations.Left-hand figures: Current possibility with line-of-sight stars Right-hand figures: Possibility with SIM.

Shown are allowed 1 & 2 sigma contours in the space of stellar velocity anisotropy (beta) and central DM density slope (gamma) as derived using an underlying correct model with a cuspy (CDM) central profile for the lower set of figures and with a cored (WDM) central profile for the upper set of figures.

The Left-hand figures utilize 1000 line-of-sight stars observed in a mock dwarf galaxy. xs denote the correct answer. NOTE: The current state-of-the art is 200 LOS stars, but we expect the situation to get better in the next few years.

The right-hand figures utilize 1000 LOS stars + 200 stars from SIM. Left hand figure: error in determined central density slope as a function of the number of SIM stars given 1000 LOS velocity measurements. We assume ~7km/s errors in the proper motions. (right) The black lines are the luminosity functions (using only CMD+2CD giantcandidates). For both Draco and UMi we added the Horizontal branches and that resulted in the dotted black lines.The colored-lines represent total integration time (the right y-axes) for a given magnitude limit: magenta, blue, red and green linesrepresent 10, 7, 5 and 3 km/s uncertainty in the proper motion numbers. So, for a given total allocation time with SIM (the righty-axes), and for a given targeted uncertainty (color of the line), you can read what your magnitude limit is in the x-axes, and then for thatmagnitude limit you can read the number of stars you can do in the left y-axes.

Right hand figure: Colored -- The black curve shows the counts of stars (left ordinate) selected to be members of each dSph for different limiting magnitudes. The colored lines show the amount of integration time (right ordinate) from the SIM time estimator to observe all stars to a given magnitude achieving proper motion precision corresponding to transverse velocities of different accuracy levels (assuming a 5 year mission and the typical number of visits). Green is 3km/s accuracy, red is 5 km/s, blue is 7 km/s and magenta is 10 km/s.Since the work of Searle & Zinn which showed that the halo globular clusters exhibit a significant age spread. Late infall is also a key element of more modern theories for the formation of structure in the universe in the presence of Cold Dark Matter where gals formed of subunits, continue to fall together until late timesWe can apply this idea to the Sgr stream. Here we look at stars in the Sgr trailing arm and see the the velocity spread only gradually thickens with length along the arm. If ALL from scattering.When this more detailed evolution at high resolution is coupled to the CDM models at larger scales, we can see how the EVOLUTION of a MW-like halo can continue at later times with the Milky Way shredding and INGESTING a number of infalling satellites.

Yes - its a surface brightness thing - note that most of our plots go down to 38 mag/arcsec. Statement is 'of order 1 stream brighter than 30th mag/arcsec'.... less than 1 actually attached to a still-bound satellite this bright.(Johnston et al. 2007, in prep).